1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to repair or alteration of lithographic printing plate constructions that may be imaged on- or off-press using digitally controlled laser output.
2. Description of the Related Art
In offset lithography, an image to be transferred to a recording medium is represented on a plate, mat or other printing member as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a dry offset printing system, the member is simply inked and the image transferred onto a recording material; the member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is brought into contact with the blanket cylinder by an impression cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the printing member prior to or in conjunction with inking. The ink-rejecting fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
U.S. Pat. Nos. 5,339,737 and 5,379,698 disclose a variety of, inter alia, dry lithographic plate configurations for use with imaging apparatus that operate by laser discharge. These plates may be imaged on a stand-alone platemaker or directly on-press, and feature silicone or fluoropolymer surface layers of sufficiently low surface energy to repel ink. Accordingly, ink may be applied to these plates directly, without the use of fountain solution as an abhesive agent. More traditionally, silicone-surfaced plates have been prepared for imaging by photoexposure. The silicone surface layer is typically a photopolymerizable polyorganosiloxane, selective exposure of which to actinic (usually ultraviolet) radiation cures the material in an imagewise pattern; unexposed portions are removed by a photographic development process. See, e.g., U.S. Pat. Nos. 4,019,904 and 4,853,313.
The acceptability of the ultimate printed image, of course, depends on more than accurate imaging; the printing member must be free of surface and structural imperfections that themselves mar the imagewise pattern. Particularly in the case of traditional, planar constructions that are individually taken from storage and mounted to plate cylinders, printing members are vulnerable to damage at numerous handling stages.
One type of plate damage results from deposition of contaminants that alter the affinity characteristics of the affected area. For example, oily contaminants deposited onto the hydrophilic area of a wet plate, or onto the oleophobic area of a dry plate, will attract ink and cause its application to areas of the recording medium that should remain blank. Such contaminants can originate with a variety of sources--even from fingerprints left by the press operator in the course of handling and mounting the plate. Surface scratching can produce a similar reversal of affinity. For example, damage to the oleophobic surface of a dry plate exposes the underlying oleophilic layer, thereby causing inappropriate application of ink to the recording medium.
Deletion fluids have been developed for gross correction of such damage. In a wet-plate system, such fluids typically etch away chemically the ink-receptive contaminant, re-exposing the underlying hydrophilic layer. For dry plates, silicones in liquid form are applied to plate areas rendered improperly ink-receptive, thereby re-establishing oleophobicity. It is also possible to create, rather than delete, ink-receptivity in selected plate areas to ameliorate imaging errors or other forms of damage. Obviously, fluids used to alter plate affinity are most easily applied to relatively large non-image areas.
Although digital imaging avoids the most cumbersome aspects of traditional platemaking, plates imaged off-press still must be manually (and sequentially) loaded onto the platemaker, imaged, inspected, then transferred to the press and mounted to their respective plate cylinders. Even plates that are imaged on-press must be withdrawn individually from their packaging and transferred to the press. Any of these handling operations can result in affinity-altering contamination.